A 5-inch gun is a gun with a 5-inch bore. Examples include these naval weapons:
BL 5 inch gun Mk I – V British naval and coast defence guns of 1880s-1890s 5-inch gun M1897 and M1900 - US Army coast artillery weapons 1900-1920 5"/25 caliber US anti-aircraft gun 5"/40 caliber gun - USN gun 5"/50 caliber gun - USN gun 5"/51 caliber US anti-ship gun 5"/38 caliber US dual-purpose gun 5"/54 caliber Mark 42 gun US dual-purpose gun 5"/54 caliber Mark 45 gun US (and 5"/62 caliber Mark 45 Model 4 gun) Oto Melara 127/54C dual-purpose gun Italian naval gun Oto Melara 127/64 dual-purpose gun Italian naval gun